Diazonium salts are compounds having very high chemical activity, and react with phenol derivatives or compounds having active methylene groups, called coupling components, to readily form azo dyes. They are also photosensitive, and decompose by light irradiation to lose their activity. Diazonium compounds therefore have long been utilized as light recording materials represented by diazo copies (see Shashin Kogaku no Kiso (Higinen Shashin) [The Fundamentals of Photographic Engineering (Nonsilver Photograph)], pages 89 to 117 and 182 to 201 (1982), Corona Publishing Co. Ltd.).
Furthermore, by utilizing the property of light decomposition and the resulting loss of activity, recently, diazonium compounds have also been applied to recording materials which require image fixing. In this regard, light-fixing type thermal recording materials have been proposed in which the recording materials comprising recording layers containing diazonium salts and coupling components are heated in accordance with image signals to react the same and thereby form images. This is followed by light irradiation to fix the images (Koji Sato et al., Gazo Denshi Gakkaishi, 11(4), 290-296 (1982)).
However, in such recording materials using a diazonium salt as a color developing element, the diazonium salt has extremely high chemical activity. As a result, the diazonium salt gradually pyrolyzes even in the dark to lose its reactivity. Therefore, the above mentioned recording materials have a short shelf life.
Various methods have been proposed for reducing the instability of the diazonium compounds, and one of the most effective means is to encapsulate the diazonium salt. Such micro-encapsulation isolates the diazonium salt from decomposition enhancers such as water and basic compounds. Accordingly, decomposition of the diazonium salt is considerably inhibited and the shelf life of recording materials containing an encapsulated diazonium salt is also remarkably improved (Tomomasa Usami et al., Denshi Shashin Gakkaishi, 26(2), 115-125 (1987)).
In a general method of encapsulation, the diazonium salt is dissolved in a hydrophobic solvent (oily phase). The resulting solution is added to an aqueous solution of a water-soluble polymer (aqueous phase), followed by emulsifying and dispersing with a homogenizer or the like. A monomer or prepolymer for forming microcapsule walls is previously added to either the oily phase or the aqueous phase, or to both. This induces a polymerization reaction or deposits a polymer at the interface of the oily phase and the aqueous phase to form polymer walls and thereby prepare the microcapsules.
These methods are described in detail, for example, in Tomoshi Kondo, Microcapsules, Nikkan Kogyo Shinbunsha (1970), Tamotsu Kondo et al., Microcapsules, Sankyo Shuppan (1977).
The microcapsule walls thus formed may comprise various materials such as crosslinked gelatin, alginates, cellulose derivatives, urea resins, urethane resins, melamine resins and nylon resins.
When the microcapsules have walls formed of a material having a glass transition temperature that is a little higher than room temperature, such as urea resins and urethane resins, the capsule walls are mass non-permeable at room temperature but are mass permeable at the glass transition temperature or higher. Such microcapsules are called thermally responsive microcapsules and are useful for thermal recording materials.
That is, recording materials comprising a support having provided thereon a thermal recording layer containing: thermally responsive microcapsules containing a diazonium salt; a coupler; and a base allow for stable retention of the diazonium salt for a long period of time, easy formation of color-developed images by heating, and also image fixing by light irradiation.
As described above, micro-encapsulation of the diazonium salt makes it possible to far improve the stability of the thermal recording materials.
However, when the diazonium salt itself is chemically unstable, the improvement in stability is limited even if the diazonium salt is micro-encapsulated. For improving the stability of thermal recording materials, it is also important to improve the stability of the diazonium salt itself. In thermal recording materials using a conventional diazonium salt, the recording material is irradiated with light having an absorption wavelength for the diazonium salt after thermal printing. This causes photolysis or fixing of the diazonium salt such that the reactivity thereof with a coupling component is lost. Accordingly, storage of the thermal recording material in an illuminated room for a long period of time advances photolysis of the diazonium salt, which in turn tends to decrease the developed color density after storage. In view of the above, the present inventors have conducted extensive investigations relating to the photolysis of diazonium salts.